The present invention relates to methods and apparatus for the production of semiconductor ribbons from a melt and more particularly to improved apparatus and methods in which surface smoothness is improved by use of water vapor.
A reference known to the Applicants and believed to be relevant to the present invention is U.S. Pat. No. 3,605,863 issued to King on Sept. 20, 1971. This patent is hereby incorporated by reference for its teaching of apparatus and methods for formation of ribbon materials which are generally applicable to formation of semiconductor ribbons.
It is generally recognized that a major portion of the cost of production of photovoltaic solar cells is the cost of the original high purity silicon and its preparation into wafers of suitable size and shape. A standard process involves the growing of monocrystalline boules using the Czochralski technique and then mechanically sawing the boules into thin slices or wafers. Half of the original silicon material is typically lost in the sawing process. Photovoltaic cells have also been made from polycrystalline silicon after it was poured into essentially brick-shaped molds and then sawed into square slices. While such processing of polycrystalline material is less expensive than growth of single crystal material, the resulting cells have lower efficiency and essentially half of the material is still lost in the sawing process. Much effort has, therefore, been made to develop apparatus and techniques for producing wafers or ribbons of silicon directly from the molten state. The above-referenced King patent illustrates one type of apparatus which has resulted from such efforts.
The basic method illustrated by the King patent includes the contacting of a molten body of material with the surface of a moving drum or wheel. The surface of the wheel is cool relative to the molten mass and by cooling a portion thereof, pulls or drags off a film or ribbon of the material in a solidified form. In applying this type of technology to semiconductor materials, we have found that the ribbon produced generally has a very rough upper surface. The roughness is generally in the form of discrete bumps which interfere with or prohibit processing of the materials into photovoltaic cells. While it is possible to grind or polish the upper surface to a suitable smoothness, such additional operations defeat the cost reduction advantages of the ribbon methods.